Allow thread-pool.h to work without threads

thread-pool.h requires CXX_STD_THREAD in order to even be included.
However, there's no deep reason for this, and during review we found
that one patch in the new DWARF indexer series unconditionally
requires the thread pool.

Because the thread pool already allows a task to be run in the calling
thread (for example if it is configured to have no threads in the
pool), it seemed straightforward to make this code ok to use when host
threads aren't available at all.

This patch implements this idea.  I built it on a thread-less host
(mingw, before my recent configure patch) and verified that the result
builds.

After the thread-pool change, parallel-for.h no longer needs any
CXX_STD_THREAD checks at all, so this patch removes these as well.

gdb/maint.c

@@ -41,16 +41,13 @@
 #include "maint.h"
 #include "gdbsupport/selftest.h"
 #include "inferior.h"
+#include "gdbsupport/thread-pool.h"
 
 #include "cli/cli-decode.h"
 #include "cli/cli-utils.h"
 #include "cli/cli-setshow.h"
 #include "cli/cli-cmds.h"
 
-#if CXX_STD_THREAD
-#include "gdbsupport/thread-pool.h"
-#endif
-
 static void maintenance_do_deprecate (const char *, int);
 
 #ifndef _WIN32